Fetching BLOB Data Using IRow::Open and ISequentialStream

Fetching BLOB Data Using IRow::Open and ISequentialStream

IRow::Open supports only DBGUID_STREAM and DBGUID_NULL type of objects to be opened.

The following function uses IRow::Open and ISequentialStream to fetch large data.

Large data can be bound or retrieved by using the ISequentialStream interface. For bound columns, the status flag indicates if the data is truncated by setting DBSTATUS_S_TRUNCATED.

void InitializeAndExecuteCommand()
{
    ulong iidx;
    WCHAR* wCmdString=OLESTR(" SELECT * FROM MyTable");
    // Do the initialization, create the session, and set command text
    hr=pICommandText->Execute(NULL, IID_IRow, NULL, 
                         &cNumRows,(Iunknown **)&pIRow)))
    //Get 1 column at a time
    for(ulong i=1; i <= NoOfColumns; i++)
      GetSequentialColumn(pIRow, iidx);
    //do the clean up
}
HRESULT GetSequentialColumn(IRow* pUnkRow, ULONG iCol)
{
    HRESULT hr = NOERROR;
    ULONG cbRead = 0;
    ULONG cbTotal = 0;
    ULONG cColumns = 0;
    ULONG cReads = 0;
    ISequentialStream* pIStream = NULL;
    WCHAR* pBuffer[kMaxBuff];//50 chars read by ISequentialStream::Read()
    DBCOLUMNINFO* prgInfo;
    OLECHAR* pColNames;
    IColumnsInfo* pIColumnsInfo;
    DBID columnid;
    DBCOLUMNACCESS column;

    hr = pUnkRow->QueryInterface(IID_IColumnsInfo, 
                            (void**) &pIColumnsInfo);
    hr = pIColumnsInfo->GetColumnInfo(&cColumns, &prgInfo, &pColNames);
    //Get Column ID
    columnid = (prgInfo + (iCol - 1))->columnid;
    //Get sequential stream object by calling IRow::Open
    hr = pUnkRow->Open(NULL, &columnid, DBGUID_STREAM, 0, 
                    IID_ISequentialStream,(LPUNKNOWN *)&pIStream);
    ZeroMemory(pBuffer, kMaxBuff * sizeof(WCHAR));
    //Read 50 chars at a time until no more data.
    do
    {
        hr = pIStream->Read(pBuffer, kMaxBuff, &cbRead);
        cbTotal = cbTotal + cbRead;
        //Process the data
    } while(cbRead > 0);
// do the clean up
    return hr;
}

To fetch large data using IRow::GetColumns (or IRow::Open) and ISequentialStream